1. Technical Field
The present invention relates to a novel aromatic copolymer. More particularly, the present invention is concerned with an aromatic copolymer comprising a plurality of aromatic copolymer chains, each comprising recurring 2,6-diphenylphenol units and specific phenolic comonomer units in a specific ratio, wherein the aromatic copolymer has a weight average molecular weight of from 1,000 to 3,000,000.
The aromatic copolymer of the present invention has excellent electrical characteristics (such as a low dielectric constant), low water absorption properties, an excellent heat resistance, excellent film-forming properties and an excellent adhesion to other materials, so that the aromatic copolymer of the present invention can be advantageously used as a material for an interlayer dielectric film for an LSI multilevel interconnect; a material for an LSI passivation film; a material for a substrate, such as a substrate for a printed circuit board, a ball grid array (BGA) and a multi chip module (MCM), and the like.
2. Background Art
A polymer used as a material for electric or electronic parts is required to have a high heat resistance, a low dielectric constant, low water absorption properties, an excellent adhesion to other materials and the like. Examples of polymers conventionally used as materials for electric or electronic parts include a phenolic resin, an epoxy resin, a polyimide resin, a fluoro resin, a bismaleimide resin and a polyphenylene ether resin. However, with respect to phenolic resins and epoxy resins, not only is the heat resistance low, but also the dielectric constant is disadvantageously high. Polyimide resins exhibit high heat resistance. However, polyimide resins exhibit high polarity, so that the water absorption properties thereof is high. Further, the dielectric constant thereof is disadvantageously high. Fluoro resins exhibit low dielectric constant and low water absorption properties, but have a poor adhesion to other materials. In recent years, the use of a bismaleimide resin and a polyphenylene ether resin as a material for a dielectric of a circuit board has been reported. These resins exhibit a low dielectric constant, but have a poor heat resistance. A silicon oxide film; which is currently used as a material for a dielectric of an LSI multilevel interconnect, is occasionally doped with fluorine so as to lower the dielectric constant thereof. However, there is a trade-off relationship between the dielectric constant and heat stability of a fluorine doped silicon oxide film, so that the lowering of the dielectric constant of the fluorine doped silicon oxide film is limited.
Among the polyphenylene ether resins, a poly-2,6-dimethylphenol, which is one of the engineering plastics, has been produced on a commercial scale and used in various fields. For example, a printed circuit board, which is based on a poly-2,6-dimethylphenol, is proposed. A poly-2,6-dimethylphenol resin has a low dielectric constant and low water absorption properties and, hence, this resin is useful as a material for electric or electronic parts. However, the heat resistance of this resin is not satisfactory, as compared to that of a polyimide resin or the like. The pendant methyl groups contained in a poly-2,6-dimethylphenol are likely to be oxidized by heating, and this is considered to be the reason for the poor heat resistance of a poly-2,6-dimethylphenol. In this situation, as a polyphenylene ether resin which has a high heat resistance as compared to that of a poly-2,6-dimethylphenol, a poly-2,6-diphenylphenol has been proposed and studied. It is known that this polymer exhibits a glass transition temperature of 230.degree. C., a melting point of 480.degree. C. and a thermal decomposition temperature of 515.degree. C. and, therefore, this polymer is a highly potential material for electric or electronic parts {Makro-molecules, 4, 5, 643 (1971)}. However, a poly-2,6-diphenylphenol is a crystalline polymer, so that when this polymer is heated, crystallization of this polymer proceeds. Hence, when a film of a poly-2,6-diphenylphenol is formed for use as a dielectric layer for a circuit board, the film is likely to be disadvantageously deformed or crazed by the heating operation for the formation of the film or due to the heat produced during the use of the circuit board comprising the film as a dielectric layer. Even if the film does not suffer such deformation or crazing, the film is likely to be shrinked due to the heat produced during the use of the circuit board comprising the film as a dielectric layer, so that a disconnection or the like of the circuit of the circuit board is likely to be disadvantageously caused by a stress generated due to the shrinkage. Therefore, in order for a poly-2,6-diphenylphenol to be suitable for practical use, this resin needs to be rendered amorphous.
A typical example of a method for rendering a crystalline polymer amorphous is to lower the regularity of the structure of the polymer. The lowering of the regularity of the structure of a polymer can be achieved by using a technique of copolymerization or chemical modification. With respect to a poly-2,6-diphenylphenol, A. S. Hay et al. disclosed, in Journal of Polymer Sci., Part A, vol. 31, 2015 and the like, that an amorphous poly-2,6-diphenylphenol can be obtained by copolymerization of 2,6-diphenylphenol with a comonomer which is a derivative of 2,6-diphenylphenol in which the pendant benzene ring has a substituent. However, several steps of reactions are required for synthesizing such a comonomer, so that it is difficult to obtain a copolymer suitable for practical use. Further, most of such comonomers contain an aliphatic group or a fluorine atom, so that the synthesized copolymers cannot satisfy the required properties, such as a heat resistance, an adhesion to other materials and the like.